1. The Field of the Invention
The present invention relates to a method of continuously producing flat glass, such as sheet glass or window glass, by rolling, in which a fluid glass sheet is molded or shaped in a forming zone between at least two porous shaping rollers and conveyed by means of conveying rollers from the forming zone.
2. Related Art
The float glass process has the greatest versatility from among the many different methods of flat glass production, since the flat glass produced with that process has an excellent surface quality. However it also has the disadvantages that the investment costs for a float glass plant are very high and that one side of the float glass produced is contaminated with tin ions from the float bath, which is troublesome in many applications.
The glass quality, which was achievable by means of the float process, was not achieved by the drawing process, which was developed during the first quarter of the last century. Furthermore among other disadvantages these methods were susceptible to maintenance problems and sensitive to disturbance of the glass composition and temperature guidance and control in the drawing chamber.
The rolling method, also called the casting method, is one of the oldest methods for flat glass manufacture. In the rolling method a glass sheet flowing from a glass melt tank is formed or shaped between two cooled shaping rollers (calibrated) and subsequently is transported by means of conveying rollers into a cooling oven. Under these conditions a bulge is formed in the glass surface upstream of the upper shaping roller, the so-called upper roller. The apparatus for performing the method is simple, but has the disadvantage that the resulting glass surface has an image or imprint, which more or less matches the shape of the surfaces of the shaping rollers. This generally damages the surface quality, but it can be desired as in ornamental glass manufacture. Prior to the introduction of float glass the raw glass for mirror manufacture was produced first by rolling and subsequently by grinding and polishing both sides. Currently wired or armored glass and ornamental glass, which is made by shaping with a profiled or shaped lower roller, are the typical rolled glass products.
The surface temperature of the shaping roller cannot be freely selected. A surface temperature that is too high leads to adherence of the glass on the roller. However a surface temperature that is too low leads to too strong cooling of the glass and to breakage of the glass between the shaping rollers. On the other hand insufficient cooling leads to a too soft sheet and has a negative effect on the dimensional stability of the flat glass, especially its thickness. Thus the plant performance (flat glass throughput) has an upper limit determined by the cooling power realized under these conditions. A high throughput can be achieved with large roller diameters. For example, a throughput of about 7 m min−1 of flat glass can be achieved with a glass sheet thickness of about 3 mm with a roller diameter of over 0.4 m. With smaller roller diameters of less than 0.2 m the throughput drops to less than half of this value. Generally it can be said that the sheet speed with constant glass throughput (amount) is inversely proportional to the thickness of the sheet. Repeated attempts to improve the surface quality of rolled glass are based on the low apparatus expenses associated with the rolling process. It has been long known to produce glass articles of outstanding surface quality by blowing a rotating glass gob in paste form. In this latter method the glass surface slides on a gas film between the glass and a forming body, which is produced by evaporation of water from the paste.
This principle has already been transferred to roller glass manufacture. Thus e.g. from JP (A) 2001-180949, it is known to shape a glass sheet between three porous roller pairs, which are acted on from the interior with an evaporating liquid, water. The liquid evaporates at the working temperature of the rollers and the vapor escapes through the pores. The glass sheet is then prevented from coming into contact with the roller material and its surface defects because of the vapor cushions arising on the outside of the rollers.
U.S. Pat. No. 3,137,556 A discloses a method, in which the glass sheet is shaped between only two porous rollers, on whose surfaces gas cushions are formed.
WO 2004/000 738 A1 teaches a similar method, which likewise manages with a roller pair. The liquid glass sheet is conducted in an S-shaped manner over the rollers, and a gas cushion prevents contact of the rollers with the glass sheet. Subsequently the glass sheet is conducted to a conveyor belt, on whose surface a gas cushion is similarly formed, in order to prevent contact of the glass surface with the conveyor belt.
Flat glass with a better surface quality may be made with this method, whereby a surface quality of fire-polished glass may be obtained. However the circumstance that the gas and vapor in this method have a clearly poorer thermal conductivity than metallic or ceramic roller material is disadvantageous. As a result the glass sheet is cooled slower than it would if it were in contact with the generally metallic roller material. Thus a more expensive guidance of the viscous glass sheet by several roller pairs acted on with a gas cushion (JP(A) 2001-180949) or by a conveyor belt provided with a gas cushion (WO 2004/000 738 A1) is required. Also a longer cooling path must be selected, which is not always easy and is associated with contingent rebuilding of the existing apparatus and in each case with higher costs. Above all, it is not always possible to control the frequently occurring slipping, Which impairs the operation of the plant and because of that can lead to quality problems.